During fetal life an uphill gradient for calcium ions exists between maternal and fetal circulations (maternal Ca2+ 1.95 mEq/1, fetal Ca2+ 2.45 mEq/1). We postualte that this gradient is maintained by an active transport system for calcium ions, similar to the calcium pump in renal tubular and intestinal mucosal plasma membranes. To characterize the enzyme, placental plasma membranes from guinea pigs were prepared according to the method of Post and Sen (Methods Enzymol. 10: 762, 1967). Samples were incubated for 30 min at 37 C in solutions containing 70 mM Na+, 20 mM Tris (pH 7.6), Ca2+ or both, in concentrations varying from 0.025 mM to 20 mM, and 5 mM Na2ATP. P1 and protein were determined and results expressed as umole P1 released per mg protein in 30 min. Ca2+ in the absence of Mg2+ stimulated P1 production. Mg2+ in the absence of Ca2+ also stimulated the enzyme but to a lesser degree. 5 mM Ca2+ produced maximal stimulation (15–25 μmole P1/mg protein in 30 min). Mn2+, but not Sr2+, stimulated P1 production, as with other Ca2+ ATPases (renal, intestinal mucosal). The pH optimum was 8.2; at 7.2 and 9.5 the enzyme activity was 50% of the maximum. Ouabain (1 mM) was not inhibitory, but addition of increasing amounts of EDTA led to progressive loss of acitivity; total inhibition occurring at 5 mM EDTA. Further fractionation of samples with sucrose-gradient centrifugation doubled the specific activity of the enzyme in the plasma membrane fraction. We believe that this enzyme of the placental plasma membranes activates a calcium pump which maintains the gradient of calcium ions between maternal and fetal circulations and ensures normal calcification in the fetus.